EP0781828A1 - Continuous pyrolysis and decoking process, paticularly applicable to the production of acetylene - Google Patents

Abstract

Continuous pyrolysis and de-coking is carried out in a reaction zone (40), of a refractory material, aligned in one axial direction, which comprises a heating zone and subsequent cooling zone. The heating zone consists of at least 2 rows (1,2) parallel to the axis and separated by a partition (70) of refractory material between two successive rows, at least one of the rows (1) connecting with a supply (10,60) of a gaseous mixture containing a hydrocarbon (at least 1C) and water vapour, and the other (2) being connected to a non-hydrocarbon fluid (60) containing water vapour. The rows also comprise heaters (8) enclosed in sheaths (7) perpendicular to the axis of the reactor. The gaseous mixture is arranged to circulate in at least one row in the heating zone for pyrolysis to be effected and for the temperature to be at least 850 degrees C, at the exit of the zone; the water vapour is circulated in other rows at a similar temperature in a de-coking operation. Hydrocarbons and a de-coking effluent are recovered.

Description

The invention relates to a process for pyrolysis of a hydrocarbon feedstock with at least one carbon atom and simultaneously a process for decoking the coke deposited on the walls of the reactor.

It applies in particular to the continuous production of acetylene or acetylene compounds such as methyl acetylene.

The prior art is illustrated in particular by patents US-A-3,641,190, US-A-1,470,359, EP-A-0 591 856, EP-A-0 143 486, EP-A- 0 542 597 and EP-A- 0 539 270.

In high temperature thermal transformation processes of hydrocarbons having at least one carbon atom, for example pyrolysis between 900 and 1500 ° C or steam cracking around 850 ° C at the end of the heating zone, coke is formed and is deposited on the surface of the reactor walls. The reactor is then decoked, which is usually done in air at temperatures generally below 900 ° C., in the case of metal ovens, to avoid any overheating or hot spots detrimental to the good behavior of the tubes. of the oven. This exothermic decoking therefore involves stopping the entire unit and above all disconnecting the furnace from the downstream heat exchangers, which reduces the total productivity of the unit. In addition, the safety rules impose the dismantling of the hydrocarbon introduction lines and their replacement by air introduction lines, which requires a very long shutdown of the unit.

When reassembling the unit for the pyrolysis phase, the same drawbacks remain, to which is added the need to purge the reaction zone and the lines with an inert gas.

The pyrolysis of hydrocarbons with at least one carbon atom making it possible to obtain olefinic or acetylenic hydrocarbon compounds has been described, in particular in the patent applications of the applicant FR 2715583 and FR 2732014 incorporated as references.

In particular, pyrolysis reactors made of ceramic material have been used in which non-watertight partitions advantageously made of ceramic material determine the channels in which the charge and the reaction effluents circulate. These partitions advantageously have a shape adapted to create turbulence and comprise, for example, cells or cavities at the level of the heating means. These are generally ducts containing an electric heater or a gas burner.

An object of the invention is to provide a method for pyrolyzing a hydrocarbon feedstock without stopping the unit to allow decoking of this unit.

Another object of the invention is to keep the temperature of the installation substantially constant during its operation, to avoid the thermal stresses which would not fail to occur, in particular when using a gas containing oxygen. for the decoking step which implements an exothermic reaction while the pyrolysis step implements an endothermic reaction.

Given the presence of non-watertight bulkheads therefore inexpensive in the reaction zone, it has been observed that it was possible to continuously implement a process for pyrolysis of a hydrocarbon feedstock and a method for decoking the reaction zone which are not penalizing.

In detail, the invention relates to a continuous pyrolysis and decoking process in a reaction zone, of refractory material, of elongated shape in a direction (an axis) comprising a heating zone and a cooling zone following the heating zone, the heating zone comprising at least two rows substantially parallel to the axis separated by a partition, preferably non-sealed, of refractory material between two successive rows, at least one of said rows communicating with a supply of a gas mixture containing a hydrocarbon containing at least one carbon atom and water vapor, at least one other of said rows communicating with a supply of a non-hydrocarbon fluid comprising water vapor, said rows comprising means for heating surrounded by ducts substantially parallel to each other and substantially perpendicular to the axis of the reactor, coke is deposited t in the reaction zone, the process being characterized in that the gas mixture is circulated in at least one row of the heating zone so as to pyrolyze said mixture and to have an outlet temperature from said zone at least 850 ° C. and said fluid comprising water vapor is circulated in at least the other row of the heating zone so as to at least partially decoke the reaction zone and to have an outlet temperature from said zone at least 850 ° C and collecting hydrocarbons and a decoking effluent.

The invention is particularly applicable in the case where the duration of pyrolysis before decoking is greater and preferably at least two times greater than the duration of decoking.

The temperature in the row or rows where the pyrolysis takes place is advantageously maintained substantially equal to the temperature in the row or rows where the decoking takes place.

The decoking of the channels with steam at very high temperature, above 850 ° C and preferably from 1000 ° C to 1400 ° C, are sufficiently oxidizing conditions at these temperatures to transform the coke and to form carbon monoxide and hydrogen.

This is particularly advantageous in the case of reactors made of ceramic material comprising non-sealed partitions. A transfer of water vapor and hydrogen can therefore take place through the wall of the row where decoking takes place towards the row where pyrolysis takes place.

It has even been observed that a transfer of hydrogen to the pyrolysis row slows down the deposition of coke thereon.

Furthermore, a transfer of water vapor from the row where decoking takes place to the row where pyrolysis takes place is not penalizing since the pyrolysis reaction takes place in the presence of water vapor. In the other direction, if hydrocarbons pass from the pyrolysis row to the row where decoking takes place, they will be in the presence of a lot of water and will be pyrolyzed into desired products.

The heating means may be electrical resistances contained in sheaths as described in the above patents or they may consist of sheaths containing a gas burner as described in the applicant's patent application (FR 2715583).

Each row may comprise at least one layer of heating means surrounded by sheaths, substantially parallel to the axis of the reaction zone, these sheaths being substantially perpendicular to said axis.

Furthermore, it has been observed that in the presence of electric heating elements contained in relatively porous and inexpensive ceramic sheaths whose sealing is not perfect, a sheath gas containing hydrogen and / or water vapor and / or an inert gas could be used and moreover could diffuse from the inside towards the outside of the sheaths without disturbing the pyrolysis reaction and without disturbing the decoking reaction.

According to a characteristic of the process, the flow rate of water vapor introduced into the decoking row can be 1.1 to 4 times greater than the flow rate of water introduced into the pyrolysis row. Most often, it is 2 to 3 times that used during pyrolysis.

In other words, during decoking, the hydrocarbon supply is cut off in the row intended to be decoked and the water flow introduced is appreciably increased so as not to cause excessive thermal disturbance in the preheating furnace. gases upstream of the reaction zone.

According to a first variant, the collected hydrocarbons and the decoking effluent can be mixed before being introduced into the cooling zone.

According to a second variant, the collected hydrocarbons and the decoking effluent are cooled separately in their respective rows, located at the level of the cooling zone and then optionally mixed.

The cooling zone is usually a zone of direct quenching by a cooling fluid, known to those skilled in the art.

Generally, the water flow rate in the pyrolysis zone is such that the water / hydrocarbon weight ratio is between 0.55 and 20 and preferably between 0.65 and 10.

The pressure is usually between 1 and 15 bar absolute (0.1 and 15 MPa) and preferably between 1 and 3 bar absolute, the weight ratio being more particularly between 0.8 and 5.

The previously heated gases are introduced into the reactor at a temperature of about 400 to about 700 ° C, preferably 500 to 650 ° C.

Usually, the final temperature at the outlet of the pyrolysis zone is greater than 850 ° C. and more particularly between 1000 ° C. and 1400 ° C.

The characteristics of the heating elements, either electric or comprising gas burners, their number, the distance between them and their configuration are described in the patents cited above.

It is the same for the sheaths protecting and isolating them from the fluids which circulate in the reactor.

These same heating elements and these same sheaths with the same characteristics and the same configurations are found both in the pyrolysis zone and in the zone (or row) where decoking with steam is carried out.

The invention also relates to a device for implementing the method.

More particularly, the pyrolysis and decoking unit comprises a pyrolysis reactor of elongated shape in a direction (an axis) comprising at least two rows substantially parallel to the axis separated by a partition, preferably not sealed, made of refractory material between two successive rows, each row comprising a plurality of heating means arranged in at least one layer of heating elements surrounded by sheaths of ceramic material substantially parallel to each other and substantially perpendicular to the axis of the reactor, at least one of the rows being connected to a feed line for a load comprising at least one valve and to a steam supply line comprising at least one valve, at least one of said rows being connected to a feed line for a non-hydrocarbon fluid containing water vapor comprising at least one valve, said pyrolysis reactor comprising m enslavement and heating modulation connected to the heating means, the unit further comprising means for cooling the effluents produced in each row.

The unit has the advantage of being safe, reliable and easy to use. It uses refractory materials and more particularly ceramic materials known to those skilled in the art such as cordierite, mullite, silicon nitride or silicon carbide.

• . les hydrocarbures aliphatiques saturés tels que le méthane, l'éthane, des mélanges d'alcanes (LPG), des coupes pétrolières telles que les naphtas, les gazoles atmosphériques et les gazoles sous vide, ces derniers pouvant présenter un point final de distillation de l'ordre de 570 °C .
• . les hydrocarbures insaturés tels que l'éthylène, le propylène, le butadiène, des mélanges d'alcanes et d'alcènes tels que éthane + éthylène, les coupes C3, C4 et C5 de vapocraquage et de craquage catalytique.

The hydrocarbon fillers which can be used are, by way of non-limiting examples:

• . saturated aliphatic hydrocarbons such as methane, ethane, mixtures of alkanes (LPG), petroleum fractions such as naphthas, atmospheric gas oils and vacuum gas oils, the latter being able to present an end point for the distillation of l '' order of 570 ° C.
• . unsaturated hydrocarbons such as ethylene, propylene, butadiene, mixtures of alkanes and alkenes such as ethane + ethylene, cuts C3, C4 and C5 of steam cracking and catalytic cracking.

From an industrial point of view, the preferred hydrocarbons come from steam cracking and are ethane which may contain ethylene in a very variable amount but generally quite low, then ethylene in a mixture with the other hydrocarbons present in the effluent of the steam cracker.

The invention will be better understood from the description of an embodiment, given purely by way of illustration but in no way limiting, which will be made below with the aid of the appended figure, which represents a longitudinal section of a reactor. along a plane parallel to the axis of the reactor.

Hydrocarbon supply lines 11, 12, 13, 14, 15, 16 controlled by valves V1, V2, V3, V4, V5 and V6 introduce into a reactor 40 for pyrolysis and decoking of hydrocarbons from a line 10 mixed with water generally in vapor form supplied by a line 60. This line distributes it in lines 17, 18, 19, 20, 21 and 22 controlled respectively by valves V7, V8, V9, V10, V11 and V12.

These various valves V1 to V12 are adapted to allow the circulation of a mixture of hydrocarbons and water vapor in a certain number of rows of the so-called pyrolysis reactor and only of steam in other rows of the so-called reactor of decoking to remove the coke which was deposited there during the pyrolysis reaction.

Lines 31, 32, 33, 34, 35, 36 transporting the mixture of hydrocarbons and water or transporting water alone, connected respectively to lines 11 and 22, 12 and 21, 13 and 20, 14 and 19, 15 and 18 and finally, 16 and 17, are preheated in the preheating oven 30 at a temperature of 400 to 700 ° C. and are connected to the pyrolysis reactor 40.

For example, the valve V1 closing the line 11, it follows that the line 31 receives only water vapor supplied by the line 22 controlled by the valve V12. On the other hand, lines 32, 33, 34, 35 and 36 receive the hydrocarbon and water mixture, all the other valves mentioned being open.

The reactor 40 is divided into longitudinal rows (1, 2, 3, 4, 5 and 6) substantially parallel to its axis. These rows are separated from one another by partitions, 70, not leaktight in ceramic material, of shape comprising cells adapted to promote turbulence inside the row and therefore to promote the reaction. These rows contain sheaths of ceramic material 7 forming a sheet substantially parallel to the axis of the reactor. These sheaths are substantially parallel to each other and substantially perpendicular to the axis of the reactor. They contain, for example, a plurality of electrical resistances 8 immersed in a sheath gas, chosen from the group formed by water vapor, hydrogen, an inert gas and a mixture of two or more of these gases.

The steam line 31 is connected with row 1. Generally, the flow rate of steam introduced into the row where decoking is carried out is increased, for example 2 to 3 times that used in the other five rows 2, 3, 4, 5 and 6 where pyrolysis takes place. These rows are therefore connected respectively to the lines of the mixture 32, 33, 34, 35 and 36.

The terminal part of the various rows of reactors, intended for pyrolysis or decoking, receives the effluents from pyrolysis or decoking and each row is connected to a direct quenching line 47, comprising an injector with a controlled flow rate, for example of ethane if the charge is ethane, which makes it possible to cool these effluents. Once cooled to 800 ° C. for example, lines 41, 42, 43, 44, 45 and 46 connected respectively to rows 1, 2, 3, 4, 5 and 6 mix the various effluents which are discharged by a line 50.

According to another mode not illustrated, the effluents can be cooled by circulating through sealed conduits arranged in the terminal part of the rows by indirect quenching and then mixed as described above.

According to another mode not illustrated, the pyrolysis and decoking effluents from rows 1, 2, 3, 4, 5 and 6 are collected by lines 41, 42, 43, 44, 45 and 46 then mixed and sent to a direct or indirect quenching zone and once cooled evacuated by line 50.

The heating elements 8 are supplied with electrical energy independently by means of a pair of electrodes not shown in the figure, pyrometric thermocouple probes not shown are housed in the spaces where the charge circulates and make it possible to automatically regulate the temperature of each heating section, by a conventional regulation and modulation device not shown in the figure, as a function of the temperature profile chosen which applies as well to the pyrolysis reaction as to that of decoking the walls of the sheaths.

Example

A reactor described according to FIG. 1 is used to crack a mixture of ethane and water vapor in order to produce acetylene.

This reactor has 6 heating rows substantially parallel to its axis and separated by partitions in the form of cells and made of ceramic material such as silicon carbide for example. Each row includes a layer of electric heating elements. The sheaths surrounding the electrical resistances are made of silicon carbide and contain a sheath gas which is nitrogen.

Five rows work in pyrolysis (n ° 1, 3, 4, 5 and 6) while only one row (n ° 2) works in decoking.

In each row in pyrolysis, 258 kg / h of ethane and 464 kg / h of steam are introduced.

In row n ° 2 operating in decoking, the valve V2 of hydrocarbons being closed, 979 kg / h of steam are sent through the valve V11.

The mixture (ethane-water) to be pyrolyzed and the decoking steam are preheated to 630 ° C and heated in a substantially linear manner to 1,200 ° C in their respective rows under an absolute pressure of 1.3 bar.

At the outlet of the pyrolysis reactor, the pyrolysis effluent is cooled to 800 ° C by direct contact with 91 kg / h of ethane at 16 ° C while the decoking effluent is cooled to 800 ° C by direct contact with 85 kg / h of ethane at 16 ° C.

After 72 hours of pyrolysis in row n ° 1, it is decided to decoke it. For this, the ethane flow is cut by the valve V1 and to avoid a disturbance of the thermal regime of the preheater and the pyrolysis oven, the water vapor flow rate is increased (valve V12) until 979 kg are obtained. / h. Simultaneously, row 2 is again supplied with 258 kg / h of ethane and 464 kg / h of steam, by opening the valve V2 and the valve V11.

The end of decoking is checked by disappearance of carbon monoxide, which is analyzed online by infrared, for example at the outlet of the pyrolysis oven.

It is found that the decoking is complete after 14 hours in each row where it is carried out and immediately goes back into a pyrolysis reaction situation for the row which has been decoked.

So there are five rows in pyrolysis and one row in decoking. There is thus produced constantly and without prolonged shutdown of the unit 450 kg / h of acetylene. The effluents from the six rows are mixed and sent via line 50 to the treatments and product separations.

Obviously, taking into account the decoking time adapted to the charge chosen, we could have had a reactor comprising 10 rows of pyrolysis and 2 rows of decoking either adjacent or separate.

Claims (12)

Process for continuous pyrolysis and decoking in a reaction zone (40), of refractory material, of elongated shape in a direction (an axis) comprising a heating zone and a cooling zone following the heating zone, the heating zone comprising at least two rows (1, 2) substantially parallel to the axis separated by a partition (70) of refractory material between two successive rows, at least one of said rows (1) communicating with a supply (10 , 60) in a gaseous mixture containing a hydrocarbon containing at least one carbon atom and water vapor, at least one other (2) of said rows communicating with a supply (60) of a non-hydrocarbon fluid comprising vapor of water, said rows comprising heating means (8) surrounded by sheaths (7) substantially parallel to each other and substantially perpendicular to the axis of the reactor, coke being deposited in the reaction zone, the p process being characterized in that the gas mixture is circulated in at least one row of the heating zone so as to pyrolyze said mixture and to have an outlet temperature of said zone of at least 850 ° C and one makes circulating said fluid comprising water vapor in at least the other row of the heating zone so as to at least partially decoker the reaction zone and to have an outlet temperature of said zone of at least 850 ° C and collecting hydrocarbons and a decoking effluent. The method of claim 1 wherein the temperature is maintained substantially equal in each of the rows. Method according to claim 1 or 2, in which the flow rate of water vapor introduced into the decoking row is 1.1 to 4 times greater than the flow rate of water vapor introduced into the pyrolysis row. Method according to one of claims 1 to 3, in which the collected hydrocarbons and the decoking effluent are mixed before being introduced into the cooling zone. Method according to one of claims 1 to 3, in which the collected hydrocarbons and the decoking effluent are cooled separately in their rows respective in the cooling zone and then possibly mixed. Method according to one of claims 1 to 5, in which direct cooling is carried out, the hydrocarbons collected and the decoking effluent using a cooling fluid. Method according to one of claims 1 to 6, in which the sheaths contain at least one gas chosen from the group formed by hydrogen, water vapor and an inert gas. Method according to one of claims 1 to 7, in which the gas mixture is circulated in at least one row of the heating zone so as to pyrolyze said mixture and to have an outlet temperature of said heating zone of approximately 1,000 to 1,400 ° C and said fluid comprising water vapor is circulated in at least the other row of the heating zone so as to at least partially decoker the reaction zone and to have a temperature of leaving said heating zone from about 1000 to 1400 ° C Method according to one of claims 1 to 8, in which each row comprises at least one layer of heating means surrounded by sheaths substantially parallel to the axis of the reaction zone. Method according to one of claims 1 to 9, applied to the production of acetylene. Method according to one of claims 1 to 9, applied to the production of methylacetylene. Pyrolysis and decoking unit for implementing the method according to one of claims 1 to 11, comprising a pyrolysis reactor (40) of elongated shape in a direction (an axis) comprising at least two rows (1, 2 ) substantially parallel to the axis separated by a partition (70) of refractory material between two successive rows, each row comprising a plurality of heating means (8) arranged in at least one layer of heating elements surrounded by sheaths (7) made of ceramic material substantially parallel to each other and substantially perpendicular to the axis of the reactor, at least one rows (1) being connected to a supply line (10) in a load comprising at least one valve (V1) and to a supply line (60) in water vapor comprising at least one valve (22) , at least one other (2) of said rows being connected to a supply line (60) in to a non-hydrocarbon fluid containing water vapor comprising at least one valve (21), said pyrolysis reactor comprising means heating control and modulation connected to the heating means, the unit further comprising means (47) for cooling the effluents produced in each row.

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